| Popis: |
Volcanic processes during and after an eruption can impact adjacent fluvial systems by high influx rates of volcaniclastic sediment, drainage disruption, formation and failure of natural dams, changes in channel geometry and changes in channel pattern. Depending on the magnitude and frequency of disruptive events, the fluvial system might “recover” over a period of years or might change to some other morphology. The goal of this study is to evaluate the preservation potential of volcanic features in the fluvial environment and assess fluvial system recovery in a probable ancient analog of a fluvial-volcanic system. The McDermott Member is the lower member of the Late Cretaceous - Tertiary Animas Formation in SW Colorado. Field studies were based on a southwest-northeast transect of six measured sections near Durango, Colorado. In the field, 13 lithofacies have been identified including various types of sandstones, conglomerates, and mudrocks interbedded with lahars, mildly reworked tuff, and primary pyroclastic units. Subsequent microfacies analysis suggests the lahar lithofacies can be subdivided into three types based on clast composition and matrix color, this might indicate different volcanic sources or sequential changes in the volcanic center. In addition, microfacies analysis of the primary pyroclastic units suggests both surge and block-and-ash types are present. Several trends can be noted: (1) there is an overall fining-upward trend seen throughout the McDermott Member in a transition from lahars and fluvial conglomerates at the base to isolated sandstone-rich channels and extensive mudrock deposits near the top of the unit, (2) there is a lateral trend change in the grain size and thickness of lahars, and the thickness of well-developed fluvial deposits, and (3) there is evidence of drainage disruption following the deposition of lahar deposits by the deposition of fine-grained silt/mudstones and sandstones which may indicate the creation of natural dams. Stratigraphic relationships and paleoflow directions suggest lahars and other volcanic sediments were transported SSE from the La Plata Mountains through one or more paleovalleys. In comparing modern and ancient volcanically-influenced environments, it is evident the thin sheets of volcanic ash which dominate modern environments have low preservation potential. Rather, the volcanic signature in the geologic record consists of: (1) lahars interbedded with fluvial deposits, (2) volcaniclastic sediment such as reworked tuff or tuffaceous sandstone, and (3) paleocurrent and provenance data. |
